1. Field of the Invention
This invention relates to a developing apparatus for developing latent images formed on a latent image carrier using electrostatic force by rendering toner particles cling as developing agents and to an image forming apparatus such as a printer or photocopier for forming images in use of this developing apparatus and, more particularly, to a developing apparatus and an image forming apparatus in which image defects such as fogs and the like are reduced.
2. Description of Related Art
With image forming apparatuses using electrophotographic technology such as photocopiers and printers, a non-magnetic single-component development process, so called, has been known in which non-magnetic materials make toner particles serving as developing agents and in which the apparatuses develop latent images formed on a latent image carrier such as a photosensitive drum. FIG. 11 shows an example of a conventional image forming apparatus using such a non-magnetic single-component development process. Around the photosensitive drum 81, provided are a charger 82 for charging the photosensitive drum 81, a light emitting device exposing section 83 for exposing the surface of the toners 87 onto the photosensitive drum 81, a transfer roller 86 for transferring the developed images on a sheet 90, and a cleaning blade 88 for removing toners remaining on the photosensitive drum.
The toners 87 are supplied onto the developing roller according to rotation of a reset roller from the cartridge. The developing roller 84 at that time is biased to a voltage of about xe2x88x92320 V, and latent image portions on the photosensitive drum 81 are set to about xe2x88x92100 V where non-latent-image portions are set to about xe2x88x92700 V. The toners conveyed by the developing roller 84 are those of charged non-magnetic toners and designed to have a prescribed negative voltage from frictions made by the reset roller 89. The toners 87 on the developing roller 84 are in contact with the blade to keep a prescribed thickness (e.g., 10 xcexcm to 40 xcexcm). The toners 87 are made to jump onto or come in contact with the latent image portions on the photosensitive drum 81 at a position where the developing drum 84 faces to the photosensitive drum, thereby developing electrostatic latent images.
The transfer roller 86 transfers the developed toners 87 on the latent image portions onto the sheet 90, and the toners are fixed subsequently to the sheet. The cleaning blade 88 scrapes the toners passing through the transfer roller 86 and remaining on the photosensitive drum. In a meantime, the reset roller 89 scrapes the toners remaining on the developing roller 84.
The printed images made by a printing system using such an electrophotographic process tend to be subject to image defects such as fogs and printing failures on fine lines. One of the reasons is that, because toners have distributed charge amounts, the toners having charges deviated from the center of the distribution do not properly contribute to image formation.
FIG. 12 is a diagram showing a charge amount distribution of the negatively charged non-magnetic single-component toners. These toners are to be used between the developing roller and the photosensitive drum, which are in the above voltage relationship; the toners are structured to be charged negatively; and the charge targeted amount Ct is a minus value. However, all the toners are not uniformly charged to have the targeted value Ct and form a Gaussian distribution as shown in FIG. 12 in respect to the charge amounts. In FIG. 12, according to the charge amount distribution of the toners, there are a distribution Fn1 located on a lower voltage side of a distribution Fp having the targeted charge amount Ct at the center of the distribution, and distributions Fn2, Fn3 on a higher voltage side of the distribution Fp. Those distributions Fn1 to Fn3 may bring image failures such as fogs from the following mechanism.
That is, force exerted to the toners on the developing roller and the photosensitive drum during development of the electrostatic latent images can be thought as a summation of, generally, electrostatic force, mirror image force, and other forces. The electrostatic force is proportioned to the charge amount of the toners; the mirror image force is a negative force (i.e., in a direction attracted from the developing roller and the photosensitive drum) proportioned to a square of the charge amount of the toners; other forces are forces such as liquid crosslinking force, van der Waals attraction, etc. which are not depending on the charge amount of the toners.
The toners contained in the distribution Fn1, or high charged toners, have the mirror image force and other forces significantly larger than the electrostatic force, and as a general rule, the toners become strongly clinging to the developing roller. With the so-called jumping development method in which toners jump, however, most of toners move back and forth between the developing roller and the photosensitive drum, and therefore, the high charged toners contained in the distribution Fn1 may jump to the side of the photosensitive drum from this influence. Because the toners thus jumping are easily made to cling to the photosensitive drum with strong mirror image force, the toners may cause image defects such as fogs. With the low charged toners contained in the distribution Fn2, the charge itself is so weak, and therefore, the electrostatic force is smaller than the summation of the mirror image force and other forces. Accordingly, such the low charged toners may cling to the non-latent-image portions on the photosensitive drum, thereby creating causes of fogs. The reverse charged toners contained in the distribution Fn3 are charged with the reverse polarity to the targeted charge amount Ct, and the reverse charged toners tend to make developments on the non-latent-image portions by the electrostatic force.
As an approach to remove the low charged toners, an art has been known in which a conductive roller biased with a direct current voltage having the reverse polarity to the toners is placed to a position facing the developing roller. Another known art has a structure that a conductive roller biased with a direct current voltage having the same polarity to the toners is placed to a position facing to the developing roller. However, those arts are no more than removing the prescribed charged toners and do not operate for removing all the charged toners: the high charged (Fn1), the low charged (Fn2), and the reverse charged (Fn3). Those arts do not regulate the voltages of the photosensitive drum, the developing roller, and the conductive roller, so that the toner removal effect may not be obtained according to the voltage relations among the drum and the rollers.
It is an object of the invention to provide a developing apparatus and an image forming apparatus for reducing at the same time high charged, low charged, and reverse charged developing agents, as described above, before the development at the latent image carrier.
In a developing apparatus as an embodiment of the invention, a developing apparatus for developing latent images includes a latent image carrier on which latent images are formed; a developing roller for developing latent images on the latent image carrier by forming a layer made of developing agents on the developing roller; and a charge amount controlling member to which a voltage having the same voltage polarity, with respect to the voltage of the developing roller, as the voltage polarity of non-latent-image portions on the latent image carrier is fed, the charge amount controlling member being disposed in facing to a surface of the developing roller on an upstream side, in a rotation direction of the developing roller, of a position at which the latent image carrier and the developing roller face to each other.
By disposing the charge amount controlling member on the upstream side, in the developing rotation direction, of the position at which the latent image carrier and the developing roller face to each other, developing agents otherwise causing fogs are captured by the charge amount controlling member before reaching the non-latent-image portions on the latent image carrier, and therefore, the apparatus can prevent image defects from occurring in advance.
With the developing apparatus according to the invention, the developing roller has a structure in non-contact with the latent image carrier or in contact with the latent image carrier. Where the developing roller is not in contact with the latent image carrier, the developing roller is biased with an alternative voltage to jump the developing agents and render the agents cling to the latent image carrier according to the alternative voltage. Before the developing agents jump to cling to the latent image carrier, the disposed charge amount controlling member removes unnecessary developing agents. Where the developing roller is in contact with the latent image carrier, the developing agents can be made to cling to the latent image carrier at a position at which the latent image carrier and the developing roller face to each other, so that the charge amount controlling member can remove unnecessary developing agents in substantially the same way.
According to an embodiment of the developing apparatus of the invention, a time average value of electric field intensity at a position that the distance between the charge amount controlling member and the developing roller becomes the shortest is no less than a time average value of electric field intensity between the developing roller and the non-latent-image portions on the latent image carrier.
By rendering the time average value of electric field intensity at the shortest distance position to the charge amount controlling member set no less than the time average value of electric field intensity to the non-latent-image portions on the latent image carrier, the developing agents can be attracted with good efficiency to the charge amount controlling member. Moreover, the voltage polarity of the charge amount controlling member with respect to the developing roller may be made to have the same polarity as that of charges given to the developing agents. The charge amount controlling member may be made of a roller member or a plate member.
In an embodiment of the developing apparatus of the invention, a developing agent collecting voltage for directing electrostatic force exerted to the developing agents clinging to the charge amount controlling member in a direction from the charge amount controlling member toward the developing roller is fed between the developing roller and the charge amount controlling member at a position that an image non-forming region forming no image exists on the latent image carrier. Therefore, before the developing agents are excessively made to cling to the charge amount controlling member, the developing agents can be effective collected from the charge amount controlling member, so that the developing agent clinging ability of the charge amount controlling member can be always maintained. The developing agent collecting voltage may be a voltage rendering the voltage polarity of the charge amount controlling member with respect to the developing roller the same polarity as the charges possessed by the developing agents clinging to the charge amount controlling member, and for example, the developing agent collecting voltage includes a direct current voltage component and an alternative current voltage component. Such selection of the polarity makes possible removals of the developing agents according to nature of the developing agents charged extraordinarily.
The image forming apparatus of the invention includes a latent image carrier on which latent images are formed; latent image forming means for forming the latent images on the latent image carrier; a developing apparatus for developing the latent images formed on the latent image carrier; and transferring means for transferring the developed images on the latent image carrier to a transfer material, wherein the developing apparatus includes a developing roller for conveying developing agents to the latent image carrier for development, and a charge amount controlling member to which a voltage having the same voltage polarity, with respect to the voltage of the developing roller, as the voltage polarity of non-latent-image portions on the latent image carrier is fed, the charge amount controlling member being disposed in facing to a surface of the developing roller on an upstream side, in a rotation direction of the developing roller, of a position where the latent image carrier and the developing roller face to each other.
The image forming apparatus of the invention has the structure that the latent images are formed on the latent image carrier and that after the latent images are developed the images on the latent image carrier are transferred to the transfer material, and by disposing the charge amount controlling member on the upstream side, in the rotation direction of the developing roller, of the position at which the latent image carrier and the developing roller face to each other, developing agents otherwise causing fogs are captured by the charge amount controlling member before reaching the non-latent-image portions on the latent image carrier. Therefore, the apparatus can prevent image defects from occurring in advance.
With such an image forming apparatus, a developing agent collecting voltage for directing electrostatic force exerted to the developing agents clinging to the charge amount controlling member in a direction from the charge amount controlling member toward the developing roller is fed between the developing roller and the charge amount controlling member where an image non-forming region that forms no image exists on the latent image carrier. Therefore, before the developing agents are excessively made to cling to the charge amount controlling member, the developing agents can be effective collected from the charge amount controlling member, so that the developing agent clinging ability of the charge amount controlling member can be always maintained.